track attachments through overwrite+rename/move via replacement_attachment_id

test plan:
 1. upload a file to a folder in a course
 2. link to the file from course content (such as a wiki page
    or assignment body)
 3. overwrite the file with another file of the same name
 4. rename the file
 5. view the course content and ensure the file link still works
 6. repeat steps 1-5, replacing step 4 by "move the file to another folder"

fixes CNVS-12359

Change-Id: I11832c307f286920174ba4731af2e88490497f92
Reviewed-on: https://gerrit.instructure.com/35677
Reviewed-by: James Williams  <jamesw@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>